The present invention relates to a method and apparatus for hardening steel and more particularly to a method and apparatus for hardening steel pipes of substantial thickness and length.
Steel is essentially an alloy of iron and carbon. Additionally, it may contain small amounts of manganese, phosphorus, or silicon, which may be added to enhance such properties as hardness, strength, ductility, and toughness.
While a trace amount of carbon is dissolved in the iron to form a constituent known as ferrite, most of the carbon in steel exists as an intermetallic compound known as iron carbide or cementite, which forms a configuration with the ferrite known as pearlite. When a pearlite carbon steel is heated to a sufficiently high temperature, known as the critical transformation temperature, a face-centered cubic lattice crystal structure, known as austenite, begins to form, dissolving substantial amounts of carbon in the steel. The transformation temperature for most steels is generally in the range of 1340.degree. F. (725.degree. C.) to 1450.degree. F. (790.degree. C.).
When austenite steel is subsequently cooled below its critical transformation temperature, it decomposes into other forms such as pearlite, bainite, martensite, or combinations thereof. These constituents determine the array of properties possessed by the steel. The formation of these constituents is a function of both the type of steel and the rate of cooling from the critical transformation temperature. Thus, the form into which the austenite decomposes, and hence the exact nature of the resulting steel, depends not only upon the initial composition of the steel, but also the sequence of cooling. At one extreme, very rapid cooling or quenching to about 450.degree. F. (232.degree. C.) and then to about 250.degree. F. (121.degree. C.) produces a very hard constituent, known as martensite. At the other extreme, slow cooling, as in ambient air, produces a coarse pearlite. Between these two extremes in cooling a wide variety of constituents may result. However, the minimum rate of cooling is often severely limited if the formation of pearlite is to be avoided.
Over the years there have developed a variety of methods to facilitate the production of large segments of steel. These methods involve the use of alloys, which alter the point at which a given constituent will form; variations in the amount of carbon, which affect the formation of martensite and therefore hardness; and specific cooling sequences and methods.
It has long been a common practice to harden steel by heat treating followed by quenching. Typically, the steel is heated above the critical transformation temperature at which it becomes austenitic and is then cooled fast enough, usually by quenching into a liquid such as water or oil, to avoid any transformation of the austenite until the steel reaches the relatively low temperature range within which it transforms to a hard, martensitic microstructure. The steel is subsequently reheated or tempered to remove the internal stresses caused by the inherent expansion of the martensite.
Martempering and austempering, which may be thought of as modifications to the traditional heat treating and quenching process, represent two widely used commercial processes.
Both martempering and austempering produce high strength steels. In martempering, rapid cooling from the critical transformation temperature is interrupted just above the martensitic transformation temperature, e.g., about 450.degree. F. (232.degree. C.), which varies according to the steel's composition. The surface of the steel is then held at a constant temperature until this temperature is equalized throughout the piece. Then it is cooled to room temperature in order to minimize cracking caused by severe differential cooling stresses set up in the brittle martensite. The steel is subsequently tempered as in regular heat treating and quenching. No bainite is allowed to form.
In austempering, the steel is quenched to a fixed temperature and held at that temperature, e.g. 500.degree. to 750.degree. F. (260.degree. to 399.degree. C.) depending on the steel, until the austenite completely transforms to bainite and the hardening transformation is complete. This process involves less total time since no additional tempering is needed. The resulting bainite structure has a higher level of toughness for a given hardness.
The general superiority in mechanical properties of austempered steel over martempered steel is shown in the Table 1 for a 0.74% carbon steel for two given temperature sequences. The data is taken from Grossmann and Bain, Principles of Heat Treatment (5th Edition 1972), p. 179.
TABLE 1 ______________________________________ Austempered Martempered Mechanical Properties Steel Steel ______________________________________ Rockwell C hardness 50.4 50.2 Ultimate strength, psi 282,700 246,700 Yield point, psi 151,300 121,700 Elongation, % in 6 inches 1.9 0.3 Reduction of area, % 34.5 0.7 Impact, ft-lb 35.3 2.9 ______________________________________
Ausforming represents a modification of martempering. In ausforming the cooling sequence is interrupted in the 600.degree.-800.degree. F. (315.degree.-427.degree. C.) temperature range and subjected to plastic deformation prior to transformation to martensite and subsequent tempering. Although only certain alloy steels are capable of undergoing ausforming, the combination of strain-hardening and quench-hardening, followed by tempering, produce a very strong product.
All of these processes suffer from a number of limitations. For example, even though carbon is inexpensive and constitutes the most important source of hardness, the carbon content of a martensitic steel is limited. As the amount of carbon increases for a given steel, the martensitic transformation temperature lowers and the martensite formed becomes harder. Since steel is less plastic at lower temperatures and so less able to accommodate the internal stresses caused by the volume changes accompanying the formation of martensite, the addition of carbon enhances the chance of cracking. Consequently, the amount of carbon and hence the maximum hardness obtainable in a martensitic steel is limited.
Additionally, exact temperature control over time is often required if specific results are to be obtained. For example, quenching, martempering, and austempering all require very rapid cooling, particularly in the temperature range around 1050.degree. to 950.degree. F. (570.degree. to 510.degree. C.) within which relatively soft pearlite would form with very little delay.
Given the importance of the cooling rate in producing the desired properties and regulating internal stresses in a given steel, the production of large pieces of steel has always presented particular difficulties, since the temperature drop at the center lags the temperature drop at the surface. The quenching of steel from its critical transformation temperature to the martensitic transformation temperature requires a rather severe cooling rate if the formation of pearlite is to be avoided. If the steel is to be austempered after initial quenching, it must be maintained within a relatively narrow temperature range above the initial martensitic transformation temperature. However, if the piece is cooled sufficiently to avoid formation of pearlite, it is often not possible to prevent significant portions of the steel from falling below the martensitic transformation temperature. Similarly, if the steel is to be subject to martempering or modified martempering, the cooling rate must be significantly slower near or through the martensitic transformation temperature range due to the high expansion and resultant internal stresses caused by the formation of martensite. Yet, regulation of the cooling rate or maintenance of a constant temperature is often difficult.
A number of processes have been developed in an attempt to address these problems. Metal alloys, such as manganese, silicon, nickel, or chromium have been added to retard the formation of pearlite to allow for a slower initial quench and to otherwise enhance the final properties of the steel. Although metal alloys and increased amounts of carbon have been used to make steel amenable to austempering in larger sections, alloys add considerably to the expense of the steel.
Additionally, the toughness and strength produced with alloys in steels having a carbon content of roughly 0.65% or less is often very close for austempering and martempering. Accordingly, austempering has not been much utilized in larger sections with alloy constructional steels having a low carbon content since martempering procudes similar toughness and strength.
Although high strength and high toughness can be achieved by austempering high carbon alloy steels, excessively long time intervals are required for complete transformation to the bainite structure. In austempering the steel must be quickly reduced to a given temperature throughout and then held at that temperature until the austenite completely transforms to bainite. Given the lag in the change between surface and internal temperatures and the need for subsequently maintaining a constant temperature as bainite transformation occurs, the size of a piece of carbon steel which may be austempered is severely limited. Rods or other shapes having an effective diameter of much more than 0.25 inches (0.64 cm) have probably not generally been effectively austempered. Similarly, austempering has not heretofore proven effective for pipes having a wall thickness of approximately 0.125 inches (0.32 cm) or greater.
A variety of quenching materials and related processes have developed in an attempt to control temperature transformations in hardening steel. As to the quenching medium, a water quench is generally preferable due to availability, reduced health hazards, effectiveness in removing scale from the surface of steel parts, and high heat capacity. However, the high rate of cooling creates and causes problems in controlling temperature, especially where larger pieces of steel are being treated. This is particularly the case where a water bath is employed.
In salt quenching, the steel is generally quenched in a salt bath at 800.degree. F. (427.degree. C.) and then cooled in air. However, salt is more expensive than water as a cooling medium and air cooling is nonuniform, thus causing hot spots leading to weak points in the steel. Additionally, the molten salt generally provides a comparatively poor quench. Although the molten salt successfully avoids any temperature drop below the temperature of the salt bath, the attainable rate of cooling is not particularly fast, especially when compared with water.
Oil quenches have been used to reduce the rate of cooling of hot steel. However, the reduced rate of cooling can result in the formation of pearlite. Additionally, oil quenching is expensive, relatively slow, and creates pollution problems.
A variety of methods and devices have been developed in an attempt to properly control heat transfer from both the exterior and interior surface of pipes while using water, as well as other substances, as a cooling medium.
A variety of methods and devices use spray nozzles to impact droplets of water against a pipe. For example, U.S. Pat. No. 3,294,599 discloses an internal quench head which works in conjunction with an external quench head to spray water against the inside and outside surfaces of the pipe, while U.S. Pat. No. 3,682,722 discloses an oil quench from rotating nozzles, which direct a stream of spray against a tubular article.
U.S. Pat. No. 4,165,246 discloses a process for heat treating steel pipes with a wall thickness ranging from 16 to 36 mm. A steel pipe is first heated over a cross section of the pipe wall to the critical transformation temperature. The pipe is then passed on rollers to a cooling zone where water from nozzles encircles the surface of the pipe to quench the surface below the martensitic transformation temperature. As the martensite is formed, heat supplied from the internal unquenched portions of the pipe or an independent source, tempers the martensite surface layer, while the unquenched internal layers form an interstage structure. The speed of the pipe through the process is greater than the critical cooling speed.
U.S. Pat. No. 4,204,892 discloses another method for heat treating steel tubes which also involves the cooling of a surface layer to form martensite followed by self-tempering due to internal cooling. In one embodiment the self-tempering step is followed by a second quench such that the properties of the center of the steel depend on the equalization temperature of the second cooling step. The second cooling step is timed such that the equalization temperature is obtained before transformation of the residual austenite into bainite.
Other processes immerse the surface in the cooling medium. For example in U.S. Pat. No. 3,623,716 there is disclosed an apparatus for hardening long pipes by passing a cooling medium, such as water, from a nozzle in a helical pattern through the inside of a hot pipe which is immersed in a bath of cooling liquid.
Another steel hardening apparatus, disclosed in U.S. Pat. No. 3,877,685, attempts to control the relative rates of cooling in the interior and exterior surfaces of a pipe. Two streams of water are respectively directed to the inside and outside surfaces of the pipe. The inside surface of the pipe is more effectively cooled due to the speed and helical nature of flow inside the pipe. After the initial quenching stage as the steel enters the martensitic transformation range, the rate of cooling is reduced by the diversion of increasing amounts of water from the inside to the outside of the pipe. A sleeve mechanism, which is located in concentric feed conduits, is used to reduce the flow to the inside portion of the pipe.
U.S. Pat. No. 2,307,694 discloses a cylindrical quenching device which directs water against the cylindrical surface of a hollow barrel as a pipe passes through the barrel.
Other patents disclose devices with a variety of quenching mechanisms using quenching baths and the like.
These and other devices and methods suffer from one or more of several limitations in addition to those already discussed. For example, none of the prior devices are suitable for quenching, martempering, austempering, and ausforming without substantial modification. Additionally, prior devices fail to make efficient use of the inherent heat of the pipe produced by the initial raising of the pipe temperature above the critical transformation temperature. Prior devices and methods using water as a quenching medium also fail to produce a steel pipe of substantial size having a carbon content of greater than 0.50%. Additionally, many prior devices fail to provide efficient heat transfer from the steel pipe. Furthermore, the thickness of a steel pipe which may be successfully austempered or martempered is generally believed to be limited to approximately one inch (2.54 cm) more or less.
These and other limitations of prior processes and methods are substantially minimized, if not eliminated, by the present invention.